ACTION OF BACTERIA 285 



pathogenic organisms, which may be inhaled, and thus gain entrance 

 to the body and set up disease. Diphtheria and pulmonary tubercu- 

 losis are two examples. In this channel of infection pathogenic 

 bacteria must, as a rule, be present in large numbers, or must meet 

 with devitalised and non-resisting tissues, to set up disease. 



These, then, are the five possible ways in which germs gain 

 access to the body tissues. The question now arises. How do 

 bacteria, having obtained entrance, set wp the process of disease ? For 

 a long time pathologists looked upon the action of these microscopic 

 parasites in the body as similar to, if not identical with, the larger 

 parasites sometimes infesting the human body. Their work was 

 viewed as a devouring of the tissues of the body. Now it is well 

 known that, however much or little of this may be done, the 

 specific action of pathogenic bacteria is of a different nature. It is 

 twofold. "We have the action of the bacteria themselves, and also 

 of their products or toxins. In particular diseases, now one and 

 now the other property comes to the front. In bacterial diseases 

 affecting or being transmitted mostly by the blood, it is the toxins 

 which act chiefly. The convenient term infection is applied to those 

 conditions in which there has been a multiplication of living 

 organisms after they have entered the body, the word intoxication 

 indicating a condition of poisoning brought about by their products. 

 It wiU be apparent at once that we may have both these conditions 

 present, the former before the latter, and the latter following as a 

 direct effect of the former. Until intoxication occurs, there may be 

 few or no symptoms; but directly enough bacteria are present to 

 produce in the body certain poisons in sufficient amount to result in 

 more or less marked tissue change, then the symptoms of that tissue 

 change appear. This period of latency between infection and the 

 appearance of the disease is known as the incubation period. Take 

 typhoid, for example. A man drinks a typhoid-polluted water. 

 For about fourteen days the bacilli are making headway in his body 

 without his being aware of it. But at the end of that incubation 

 period the signs of the disease assert themselves. Professor Watson 

 Cheyne and others have maintained that there is some exact 

 proportion between the number of bacteria gaining entrance and the 

 length of the incubation period. 



Speaking generally, we may note that pathogenic bacteria divide 

 themselves into two groups : those which, on entering the body, 

 pass at once, by the lymph or blood-stream, to all parts of the body, 

 and become more and more diffused throughout the blood and 

 tissues, although in some cases they settle down in some spot 

 remote from the point of entrance, and produce their chief lesions 

 there. Tubercle and anthrax would be types of this group. On 

 the other hand, there is a second group, which remain almost 



